How to make a shiny panel on a matt texture
Your node organization will depend on the result you want. For example where exactly do you want to see shininess?

Anyway, as your mask has a big rectangle, let's imagine that you simply want to display a silver rectangle. As this rectangle is transparent and your mask image seems to play with transparency, and not black &amp; white, it is made to use the Alpha output of the Image Texture node.

By the way, there are also grey areas on your mask, grey areas won't be useful if you play with the alpha channel, it will only be useful if you play with black &amp; white, and therefore if you use the Color output of the Image Texture, and not the Alpha. In that case, if you want to use a B&amp;W mask, your rectangle needs to be white.

So let's come back to our case: Just plug the Alpha output of your Image Texture node to the Factor of a Mix Shader. This node will mix your previous nodes organization and a new one, a Glossy, and a glossy rectangle will appear.